1. Field of the Invention
The present invention relates to a crankshaft support structure of an internal combustion engine for rotatably supporting journal portions of the crankshaft in a lower portion of a cylinder block.
2. Description of the Related Art
Many types of reciprocating engines use a structure wherein a crankshaft is rotatably assembled with a lower portion of a cylinder block having cylinder bores, and motions of respective pistons reciprocating inside the cylinder bores are converted into rotational motions.
Conventionally, in an assembly structure for a crankshaft of an engine of the type described above, for example, a saddle-shaped bearing cap is assembled to an undersurface of a wall-shaped journal support base formed between a pair of skirt portions extending down from both sides of the cylinder bore, whereby journal portions of the crankshaft are rotatably supported.
Generally, journal support bases are each formed into the shape as a wall that extends along the direction crossing with a direction of an axis (“axial direction,” hereinafter) of the crankshaft. On an undersurface of the journal support base, there are formed semicircular bearing portions wherein half parts of the respective journal portions of the crankshaft are accommodated. With bearing caps being mounted on the undersurfaces of the respective journal support bases, the journal portions of the crankshaft are rotatably supported.
Ordinarily, a reciprocating engine is designed in accordance with a variety of limit designs. As such, when the engine is continually operated at a high rotational speed zone with for example, near-maximum outputs or higher, an undesirable case can take place where a crack can occur in a journal support base. Such a case is considered to occur with application of an excessively high load, excitation force, and/or the like developed with the high rotational speed operation on the journal support base.
Generally, configurations have been proposed to provide reinforcement on the journal support base in such an event as described above. For example, in a configuration disclosed in Jpn. Pat. Appln. KOKAI Publication No. 08-277747, reinforcement is provided in such a manner as to increase dimensions such as the width dimensions of the entirety of the bearing surface forming the bearing portion and the entirety of the journal support portion having the bearing portion.
However, compactness is required for engines. As such, it is desired to prevent engines from being increased in size. However, according to such the reinforcement, the journal portion of the crankshaft is compelled to be configured with an increased width to meet the increased width dimension of the bearing portion. This results in an increase in the total length of the crankshaft, hence leading to an increase in the size of the cylinder block. Consequently, the engine has to be enlarged. Even supposing that the configuration is formed by reducing the width dimension of a pin portion of the crankshaft in consideration of the increased width dimension of the bearing portion in order to prevent the increase in the total length of the crankshaft. In this case, however, another problem takes place in that the strength of a connecting rod mated with the pin portion reduced to be insufficient.